BACKGROUND Intracellular antioxidants have been shown to be depressed during hypoxia, and recovery upon reoxygenation has been correlated with the available antioxidant reserve. To test whether these antioxidant changes are also occurring at the cardiac myocytes level, we studied changes in antioxidant enzyme activities as well as cell injury in isolated cardiac myocytes exposed to hypoxia and reoxygenation. METHODS Isolated Ca(2+)-tolerant myocytes from adult male rats were subjected to 30 minutes hypoxia and 15 minutes reoxygenation. Antioxidant enzymes superoxide dismutase, glutathione peroxidase, catalase; lipid peroxide content; electrolytes (Na+, Ca2+); morphology; and high energy phosphates (ATP, ADP, AMP, creatinine phosphate) were studied in these myocytes. The effects of exogenous catalase (40 units/ml) on hypoxia-reoxygenation induced changes in myocytes were also studied. RESULTS Hypoxia resulted in a reduction in Mn superoxide dismutase and glutathione peroxidase activities with no change in CAT activity and malondialdehyde content. Reoxygenation of hypoxic cells resulted in recovery of Mn superoxide dismutase but not in glutathione peroxidase activity. Reoxygenation was without any effect on catalase activity, but a significant increase in the malondialdehyde content was seen. Hypoxia as well as reoxygenation caused a reduction in the number of rod-shaped cells with a parallel increase in hypercontracted as well as round cells. There was a significant increase in the myocyte Na+ and Ca2+ content during both hypoxia and reoxygenation, and this was accompanied by leakage of lactate dehydrogenase into the perfusion medium. These changes due to hypoxia and reoxygenation were significantly attenuated by addition of catalase (40 units/ml). High energy phosphates ATP, ADP, and AMP declined during hypoxia, and creatine phosphate was significantly reduced during reoxygenation. CONCLUSIONS Hypoxia induces specific antioxidant changes in the isolated cardiac myocytes. Reduced ability to remove hydrogen peroxide appears to be an important determinant of myocyte injury during reoxygenation.